As it stands, star rating is at least a somewhat accurate measure of hit object density.
If mappers could decide their own star ratings, it probably wouldn't go very well. People would probably start going, "My Normal is a 3!" just because it's a Normal, even if it's more like a 4 to a 4.5.
EDIT: And BATs and MATs are more lenient about stuff than ever IMO, so I don't trust them that much. 'Sides, do we really need even more stuff for them to be worried about? They make enough mistakes with classification as it stands.